Following up on r1603485, simplify the 'current' contents parsing within the
transaction commit.  See http://svn.haxx.se/dev/archive-2014-06/0053.shtml

* subversion/libsvn_fs_fs/transaction.c
  (get_next_revision_ids): Remove this function.
  (commit_body): Use the new svn_fs_fs__read_current() function when
    obtaining the youngest revision and, possibly, the (next-node-id
    next-copy-id) values instead of doing the same in two separate calls.
    Explicitly update the youngest revision cache, because this is what
    the svn_fs_fs__youngest_rev() function does underneath and we have
    just switched to using a parse-only utility function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1603487 13f79535-47bb-0310-9956-ffa450edef68